(a) Field of the Invention
The present invention concerns ultra thin solid state batteries and their process of preparation. More generally, the invention relates to a process for the preparation of thin polymer electrolyte batteries with lithium or sodium anode, as well as to batteries obtained by this process. More particularly, the invention relates to the preparation of polymer electrolyte batteries, from a laminated mother-battery which is sharply and mechanically cut out. In other words, the present invention concerns the use of a laminated battery of large surface area which is prepared by continuous processes, in which the design is particularly adapted to the manufacture of smaller elements by simple direct cutting out of the laminated product, as well as the process of manufacturing small ultra thin batteries by mechanical cutting out.
(b) Description of Prior Art
During the last ten years primary and recharge able lithium type batteries were the object of considerable research and development. The aim is to develop a low cost battery, with a large energy content and good electrical performances. With this in view, a large number of battery designs have been developed to comply with different applications such as micro-electronics, telecommunications, portable computers and electrical vehicles, to name only a few.
Electrochemical batteries or generators, whether rechargeable or not, are all made of an anode which may consist of a metal such as lithium, a cathode which consists of an insertion compound, reversibly alkali ions reversibly or non reversibly inserting alkaline ions, such as vanadium oxide or manganese dioxide, a mechanical separator placed between the electrodes, and an electrolytic component. The term electrolytic component means any material comprised within the generator which is used for ionic transport, except the active materials of the electrode, in which the ions Li.sup.+ may move, at the level of the separator as well as in the composite electrode. During discharge or charge of the generator, the electrolytic component ensures the transport of the ionic species through the entire generator, from one electrode to the other and even inside the composite electrode. In a polymer electrolyte battery, the functions of separator and of electrolytic components are generally combined in the same material.
Small, rechargeable or non rechargeable lithium batteries, such as button batteries and flat batteries are generally manufactured by elaboration and cutting out of individual components which are thereafter assembled In certain cases dealing with flat batteries consisting of thin films, the batteries or their components are elaborated in the form of batteries or multiple elements of predetermined dimension which are thereafter cut out in electrochemically inactive zones which are provided for this purpose so as to enable local cut out without damages for the electrochemical device. Examples of batteries utilizing this manufacturing process are describes in U.S. Pat. Nos, 4,177,330; 5,378,557 and 5,547,780.
In most cases, the active electrodes: metallic lithium and composite cathode, are directly in contact with the wrapping material which is then used as protection barrier and current collector; often these electrodes are directly prepared on the wrapping material by coating or pressing. This combination of two functions: barrier and collector on the same material, then enables to optimize the weight and the volume of the complete battery.
This procedure however has many disadvantages:
the need to predetermine the dimensions and the position of the elements or the batteries during the manufacture of multiple elements to be cut out; PA1 losses in material and in optimization associated with the protrusions and prepositioning required by the operations of cutting out and sealing of the whole of the device; PA1 problems associated with the positioning of the elements over one another during assembly or cutting out; PA1 the excess thickness required in order that the same material may be used as air and water impervious barrier as well as collector.
This excess thickness becomes particularly penalizing for batteries consisting of thin films mounted in series or parallel assemblies, i.e. when a plurality of individual batteries are superposed to develop the required voltage or amperage.
The mechanical cutting out of a polymer electrolyte battery into pieces is possible a priori, however it is generally understood that the operation of cutting out produces short circuits and leaves weak points especially when this operation is carried out with mechanical means (U.S. Pat. No. 5,250,784), which is indirectly confirmed by more complex processes of cutting out multiple batteries (U.S. Pat. Nos. 5,378,557 and 5,547,780) where cutting out is effected in non-electrochemically active predetermined zones. Many times, the inventors have carried out a mechanical cutting out of a laminated battery into thin films, by means, for example of a pair of scissors without permanent loss of the voltage of the pieces. However, these tests carried out under ambient air could not suggest a manufacturing process, since, on the one hand, the tests were based on the irreversible oxidation of lithium with water and the components of air, and on the other hand, because it is known that the mechanical cutting out itself creates zones which are favorable to the development of short circuits, for example during cycling, as a results of the mechanical cutting out which tends to draw the collectors of the anode and the cathode together.
The invention has for object the manufacture of polymer electrolyte batteries with lithium or sodium anode utilizing a surprising self-healing effect of the anode, and which facilitates the cutting out into small elements with no or little reject.
It is also an object of the invention to provide a process of cutting out a laminated mother-battery which leaves no electrochemically weak points as illustrated from an electrochemical cycling of batteries cut out according to the invention.
Another object of the invention is to provide a process of manufacturing batteries which, in a non limiting manner, relies on the chemical or electrochemical dissolution of metallic lithium or sodium when in contact with the material of the cathode so as to rationalize the self-healing mechanism.
It is another object of the invention to establish through examples that there is a mechanism of self-healing which tends to prevent short-circuits often caused by mechanical cutting out and this, even in the absence of air and water.
It is another object of the invention to establish through tests of cycling that the phenomenon of self-healing tends to eliminate lithium from the cut out zones.
Another object of the invention resides in the design of a simplified process of manufacture which is based on the sharp cutting out of a battery of large surface area prepared by continuous processes, without having in to presume in advance of the shape or the dimension of the battery to be produced.